Studies to be performed in the following year are related to the hypothesis that a tubular-vascular feedback mechanism operates between distal tubule and the afferent arteriole and serves to control the afferent arteriolar resistance. One objective is to determine the level of glomerular pressure, by use of the proximal tubular stop flow methods, in dogs which exhibit autoregulatory capability. Since this method of determining glomerular pressure results in complete interruption of distal volume delivery, it would be expected to interfere with a distal tubular feedback mechanism. Furthermore, if the distal tubular feedback mechanism is responsible for the autoregulation of glomerular pressure, in interruption of distal delivery should interfere with this phenomenon at the single nephron level. In additions, tubular perfusion studies are planned. We plan to alter the composition of the distal tubular perfusion solution, and observe the effects on the single nephron glomerular filtration rate and on the glomerular pressure of the perfused tubule. By perfusing with solutions of different sodium concentrations, we will be able to specifically test the hypothesis that changes in distal sodium delivery play the predominant role in this feedback mechanism. However, the experiments done to date indicate that changes in distal calcium delivery may be of much greater significance. Therefore, emphasis will be placed on testing the hypothesis that one of the principal factors that maybe involved in the feedback regulation of glomerular dynamics and afferent arteriolar resistance may be the distal delivery of calcium. Along with the experimental studies, work on a mathematical model of renal hemodynamics is progressing and will be continued in the coming year.